The present invention relates to a semiconductor circuit of the type which may be used in equipment such as a power converting apparatus, a driving method of the semiconductor circuit and a semiconductor device.
A power converting apparatus comprises not only switching devices, such as MOSFETs and IGBTs (Insulated Gate Bipolar Transistor), devices having a rectifying effect, such as diodes, and passive devices, such as capacitors, inductances and resistors, but also wires which have a parasitic inductance. The power converting apparatus converts power by repeating the on and off states of the switching and rectifying devices, in which in on and off states, currents are flowing through and are cut off from the devices, respectively. As a result, a voltage rising abruptly to a value much higher than the power-supply voltage is applied to the switching and rectifying devices at the time the devices undergo transition from the on state to the off state due to the effect of parasitic inductance.
In order to prevent the devices from being damaged by a high voltage (a spike voltage) generated at the switching time, traditionally, devices having a breakdown voltage greater than a value obtained by estimating the magnitude of such a spike voltage are used, or, as an alternative, the spike voltage is eliminated by means of a snubber circuit. The use of devices having a high breakdown voltage entails not only a high cost but, also an increased amount of incurred power loss, giving rise to an undesirable problem. On the other hand, the use of a snubber circuit for avoiding a spike voltage increases the number of components. As a result, the cost and the size of the power converting apparatus increase.
By the way, a spike voltage is generated at the switching time because a current that has been flowing through the switching device and the inductance abruptly decreases due to the switching operation of the switching device. Thus, if the abrupt decrease in current can be suppressed, the spike voltage can also be suppressed as well. In a typical method for suppressing an abrupt decrease in current, a Zener diode or an avalanche diode is employed in parallel to the switching device. When a voltage applied to an avalanche diode exceeds the breakdown voltage thereof, a current is allowed to flow through the avalanche diode, avoiding an abrupt decrease in current.
However, the use of such a diode gives rise to the following problem. When a current flows through an avalanche diode, the decrease in current flowing through a parasitic inductance disappears due to the fact that the resistance of the avalanche diode is all but zero once the avalanche diode has entered a breakdown state. The disappearance of the decrease in current, in turn, causes the voltage applied to the avalanche diode to decrease. As the voltage applied to the avalanche diode becomes lower than a voltage that causes the avalanche diode to enter an avalanche breakdown state, the avalanche diode enters an off state, trying to abruptly reduce a current flowing through the inductance. The attempt to abruptly reduce the current flowing through the inductance causes the avalanche diode to again enter an avalanche breakdown state in which a current can flow through it. That is to say, the voltage applied to the avalanche diode and the current flowing through it keep oscillating and the oscillation of the voltage and current is a cause of the generation of electromagnetic noise.
Another method besides the technique of using an avalanche diode is described, for example, in a document called EPE Journal, Vol. 4, No. 2, June (1994), pages 8 to 9. The method described therein is an example of techniques called dynamic clamping. According to the dynamic clamping technique, an avalanche diode is interposed between the collector and gate electrode of an IGBT, whereas a resistor is interposed between the gate and emitter electrode thereof. As the collector voltage exceeds the breakdown voltage of the avalanche diode, a current flows through the avalanche diode and the resistor, increasing the gate voltage. The increase in gate voltage causes a collector current to flow through the IGBT, preventing a large voltage from being applied to the device. In this case, none the less, a problem similar to that encountered in the technique of using an avalanche diode as described above also arises.
With the collector voltage exceeding the breakdown voltage of the avalanche diode, a voltage equal to the difference between the collector voltage and the avalanche-breakdown voltage is applied to the gate. That is to say, as the collector voltage exceeds the avalanche-breakdown voltage, the portion of the collector voltage above the avalanche-breakdown voltage is all applied to the gate.
In general, the IGBT has a collector current which is greatly changing due to a small variation in gate voltage so that, when the collector voltage exceeds the avalanche-breakdown voltage, the IGBT current increases abruptly. In the case of an IGBT having a breakdown voltage of several hundreds of volts and a rated current density of 200 A/cm2, for example, the saturated current density at a gate voltage of 15 V reaches as much as several thousands of amperes per square cm. This implies that, when the collector voltage exceeds the breakdown voltage of an avalanche diode interposed between the collector and the gate by a potential of only 15 V, the collector current can actually reach a value of several thousands of amperes. That is to say, an IGBT adopting the dynamic clamping technique exhibits a characteristic very similar to that of an avalanche diode wherein, at a certain voltage, the current abruptly increases. For this reason, this dynamic clamping technique also has the same problem as that encountered in the example of using an avalanche diode.
As described above, the techniques adopted in the conventional power converting apparatus for suppressing a spike voltage have problems of an increased amount of incurred power loss, a rising cost and generation of electromagnetic noise. It is thus an object of the present invention to provide a semiconductor circuit which is capable of solving these problems, a technique of driving the semiconductor circuit and a semiconductor device.